Recently, as electronic devices have been miniaturized, and lightened, and portable electronic devices have been widely used, research on a lithium secondary battery of high energy density has been actively performed. In a lithium secondary battery, materials allowing intercalation and deintercalation of lithium ions are used as an anode and a cathode, and organic electrolyte or polymer electrolyte is charged between the cathode and the anode. Also, when lithium ions are intercalated and deintercalated at the cathode and the anode, oxidation/reduction reactions generate electrical energy.
In a lithium secondary battery, a lithium-containing metal composite oxide having a high potential vs. lithium potential (Li/Li+) (for example, LiCoO2) has been mainly used as a cathode active material, and a carbonaceous material has been mainly used as an anode active material. Such materials enable the lithium secondary battery to have high capacity and high power. However, when the above described cathode active material having a high potential vs. lithium potential (Li/Li+) is used alone, electrons and lithium ions suddenly move from an anode to a cathode during an internal short caused by internal/external conditions. Accordingly, a large amount of current temporarily flows, and thus heat is suddenly generated within the battery, thereby causing fire or explosion of the battery.
In addition, when the cathode active material is exposed at a high temperature atmosphere or a battery temperature is increased by an abnormal operation of a battery, the cathode active material is decomposed at temperatures higher than a certain temperature, and thus generates oxygen. Accordingly, fire or explosion may occur in the battery. Also, when side reactions occur by contacting the cathode active material with nonaqueous electrolyte, an exothermic reaction may cause an explosion. Especially, when gas is generated within the battery due to the side reactions, the possibility of battery explosion is increased.